1. Field of the Invention
The present invention relates to a photodisc apparatus having a tracking servo system for enabling a beam spot to trace an information recording track with a high degree of accuracy, with means for canceling offset of the tracking servo loop.
2. Description of the Related Art
In recent years, optical information recording/reproduction apparatuses have been put to practical use, in which information is densely recorded in a recording medium by means of a light beam spot focused on the recording medium or information is read or reproduced at high speed through sensing the light reflected from the medium by means of a photosensor.
In the recording/reproduction apparatus of the kind described, it is necessary that the light beam applied to the recording medium is held in focused state and in on-track state, in order to record information at a high density and to read information which has been recorded at a high density. To this end, the recording/reproduction apparatus is usually equipped with a focus control means and a radial tracking control means. These control means detects, as a focus error signal and a tracking error signal, information concerning amount of defocus of the beam and radial deviation of the beam contained in the light reflected back from the recording medium. The focus control means operates in response to the focus error signal and the tracking error signal so as to maintain the light beam in focused and on-track states.
In the apparatus of the kind described, it is often experienced that the positional relationship between the photodisc and the optical head is changed due to reasons such as a deflection of the photodisc or misalignment between the disc driving mechanism and the optical head, with the result that an offset is caused in the tracking signal.
Such an offset causes the tracking position to be deviated from the optimum position. For instance, the position of a reading light beam spot for reading information from a track is undesirably deviated from the center of the track, often causing a reading error.
Hitherto, therefore, it has been a common measure to adjust the optical position of the optical head through an observation of the tracking signal by for example, an oscilloscope. This method, however, is too time-consuming and impractical.
Japanese Patent Application Laid Open No. 63-106003 discloses a gain control means for a tracking servo loop but does not show any method or apparatus for correcting offset of the tracking signal.